1. Field of the Invention
This invention relates to a semiconductor light-emitting device and more particularly, to a semiconductor laser device suited for use in short wavelength and high output applications.
2. Background Art
With optical recording devices such as an optical disc, a further increase in information recording amount has been demanded. In order to increase the information recording amount in optical recording devices, it is needed that the semiconductor light-emitting devices be shorter in wavelength and higher in output power.
Semiconductor laser devices have a problem in that if light absorption in coating films takes place, an optical absorption loss becomes so great that degradation of characteristics, such as a rise in operating current, is induced and heat is generated by the optical absorption of the coating film, thereby causing degradation at the facet called COD (catastrophic optical damage) and making it impossible to obtain desired optical output power. COD is a phenomenon wherein a facet of a semiconductor laser device locally absorbs light and becomes high in temperature, thereby melting and recrystallization of the semiconductor material at the facet of the semiconductor laser device.
Semiconductor laser devices have at least two facets including a light output facet called “front facet” and a light reflection facet called “rear facet”. Generally, in high power semiconductor lasers, a low reflectance coating film is formed on the front facet, and a high reflectance coating film is formed on the rear facet. Specifically, the reflectance of the coating film on the rear end face is typically 70% or higher, preferably 90% or higher. The reflectance of the coating film on the front end face, on the other hand, is determined based on the required characteristics of the semiconductor laser (Lower reflectance does not necessarily guarantee higher laser performance.) General high power semiconductor lasers, on the other hand, employ a coating film having a reflectance of approximately 3-7%, or a coating film having a reflectance of approximately 7-20% when it is necessary to prevent return of light.
The characteristics of this high reflection film are important so as to suppress the characteristic degradation of a semiconductor laser, COD and the like. The respective reflection films serve to protect the facet as well.
The high reflection film (high reflection coating) of a semiconductor laser disclosed in JP-A-Hei-6-224514 has the following configuration so as to improve a heat spreading characteristic and prevent COD at the rear facet on which the high reflection film is formed. More particularly, a Si film whose thermal conductivity is high is made thick and an Al2O3 film whose thermal conductivity is low is made thin. The high reflection film of the semiconductor laser disclosed in JP-A-Hei-6-224514 is formed in this way and is improved in heat spreading, thereby preventing COD.
The reflection film of the semiconductor laser set out hereinabove in JP-A-Hei-6-224514 relating to a conventional technique is excellent in heat spreading in the semiconductor laser. However, lowering of optical absorption of the high reflection film is not taken into account. Blue semiconductor lasers also have another problem with regard to the coating films. Since the wavelength of the laser beam of these semiconductor lasers is short, the coating films have a high optical absorption coefficient and hence tend to degrade, as compared to conventional semiconductor lasers. Further, since conventional coating films do not adequately function as passivation films, the crystalline structure of the end face portions of the laser may be degraded, resulting in a reduced COD threshold. This has inhibited high output power of the semiconductor laser.
[Patent Literature 1] JP-A-Hei-6-224514
[Patent Literature 2] JP-A-Hei-9-326527
[Patent Literature 3] JP-A-2003-264333
[Patent Literature 4] JP-A-2004-327678
[Patent Literature 5] JP-A-2002-305348
[Patent Literature 6] JP-A-2006-165478
[Patent Literature 7] JP-A-2005-175111
[Patent Literature 8] JP-A-2004-327581